likely FP miscompile from AArch64 backend

[regehr]

let's take this IR:

define float @f(float %0, float %1) {
  %3 = fcmp nnan ogt float %0, %1
  %4 = select i1 %3, float %0, float %1
  ret float %4
}

we're lowering it like so:

Johns-MacBook-Pro-2:build regehr$ ~/llvm-project/for-alive/bin/llc -o - foo2.ll
	.section	__TEXT,__text,regular,pure_instructions
	.build_version macos, 15, 0
	.globl	_f                              ; -- Begin function f
	.p2align	2
_f:                                     ; @f
	.cfi_startproc
; %bb.0:
	fmaxnm	s0, s0, s1
	ret
	.cfi_endproc
                                        ; -- End function
.subsections_via_symbols
Johns-MacBook-Pro-2:build regehr$ 

now let's look at invoking f(+0.0, -0.0). at the LLVM level, the fcmp ogt returns false, so we get -0.0 as the function return value. however, at least on my M1 Mac, I'm getting +0.0 as the result of the emitted fmaxnm when passed those same arguments.

cc @nunoplopes @stephentyrone

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: John Regehr (regehr)

let's take this IR: ```llvm define float @f(float %0, float %1) { %3 = fcmp nnan ogt float %0, %1 %4 = select i1 %3, float %0, float %1 ret float %4 } ``` we're lowering it like so: ``` Johns-MacBook-Pro-2:build regehr$ ~/llvm-project/for-alive/bin/llc -o - foo2.ll .section __TEXT,__text,regular,pure_instructions .build_version macos, 15, 0 .globl _f ; -- Begin function f .p2align 2 _f: ; @f .cfi_startproc ; %bb.0: fmaxnm s0, s0, s1 ret .cfi_endproc ; -- End function .subsections_via_symbols Johns-MacBook-Pro-2:build regehr$ ``` now let's look at invoking `f(+0.0, -0.0)`. at the LLVM level, the `fcmp ogt` returns false, so we get `-0.0` as the function return value. however, at least on my M1 Mac, I'm getting `+0.0` as the result of the emitted `fmaxnm` when passed those same arguments.

cc @nunoplopes @stephentyrone

[dtcxzyw]

Related issue: #93414

[regehr]

looks probably the same? should we close this as a dup?

[dtcxzyw]

I have confirmed that it can be fixed by #93575.